The scope of application of the torsion requirements is rather broad. For example, the damping effect of the operation of the pedal rotary axle of the exercise machine is a case in point. The damping effect is generally brought about by the belt friction force for variation of the damping value, or the magnetically conductive belt which is separated from the source of magnetic force by a distance for adjusting the magnitude of the magnetic force. Such conventional dampers as described above are generally cumbersome and complicated in construction in view of the fact that they consist of two wheels, belts, oil pressure cylinder, link rod mechanism, etc.
Once a predetermined value of torsional moment of the conventional adjustable torsion damper is set, the torsional resistances provided by the rotary shaft are always the same no matter how the rotary shaft is angularly rotated. Such a design as described above is not desirable as far as a mechanism having a dead point is concerned. For example, the ankles of exerciser's feet form a dead point relative to the crank arms of the exercise machine at the time when the crank arms are up and down vertically. As a result, the pedaling puts a strain on the exerciser. In other words, as far as a mechanism having a dead point is concerned, the desirable design should include a rotary shaft which has various values of torsional moment in accordance with changes in rotational angle of the rotary shaft.